Epoxidation
An epoxide is a cyclic ether with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale for many applications. In general, low molecular weight epoxides are colourless and nonpolar, and often volatile.
A compound containing the epoxide functional group can be called an epoxy, epoxide, oxirane, and ethoxyline. Simple epoxides are often referred to as oxides. Thus, the epoxide of ethylene (C2H4) is ethylene oxide (C2H4O). Many compounds have trivial names, ethylene oxide is called "oxirane." Some names emphasize the presence of the epoxide functional group, as in the compound 1,2-epoxycycloheptane, which can also be called 1,2-heptene oxide.
A polymer formed from epoxide precursors is called an epoxy, but such materials do not contain epoxide groups (or contain only a few residual epoxy groups that remain unreacted in the formation of the resin).
The dominant epoxides industrially are ethylene oxide and propylene oxide, which are produced respectively on the scales of approximately 15 and 3 million tonnes/year.
The epoxidation of ethylene involves its reaction of oxygen according to the following stoichiometry:
The direct reaction of oxygen with alkenes is useful only for this epoxide. It requires a silver catalyst. Other alkenes fail to react usefully, even propylene.
Many epoxides are generated by treating alkenes with peroxide-containing reagents, which donate a single oxygen atom. Outside of the laboratory scale, the main challenge with this approach is that typical peroxides are more valuable than the product epoxides. For this reason, this methodology is restricted to fine chemical applications.
Metal complexes are useful catalysts for these reactions. Typical peroxide reagents include hydrogen peroxide, peroxycarboxylic acids, and alkyl hydroperoxides (generated in-situ or preformed). In specialized applications, other peroxide-containing reagents are employed, such as dimethyldioxirane. Depending on the mechanism of the reaction and the geometry of the alkene starting material, cis and/or trans epoxide diastereomers may be formed. In addition, if there are other stereocenters present in the starting material, they can influence the stereochemistry of the epoxidation relative to them. The metal-catalyzed epoxidation was first explored using tert-butyl hydroperoxide (TBHP) as a source of an O atom. Association of TBHP with the metal generates the active metal catalyst with a peroxy ligand (MOOR), which then transfers an O center to the alkene.
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